1. Field of the Invention
The invention relates to a femoral slideway, and more specifically to a femoral slideway/femur-size template arrangement and a knee endoprosthesis system with such a femoral slideway.
2. Description of the Related Art
A femoral slideway of this kind is disclosed, for example, in the German patent DE 40 41 002 C2. In the surgical technique customarily used for knee-joint replacement by means of such a femoral slideway, an equal amount of bone is removed from the two condyles of the femur, so that the anterior or ventral cut is parallel to the posterior or dorsal cut. When the implant is in the position thus defined, the axis of rotation of the implant no longer coincides with the axis specified by the arrangement of the collateral ligaments, and this position is not anatomical inasmuch as when flexed, the implant is seated either too tightly on the medial side or too loosely on the lateral side.
The remedy that was recommended some time ago, namely an outward rotation of the cutting guide such that in the posterior region less bone is removed laterally than medially, whereas anterior-laterally more bone is removed than on the anterior-medial side, also presents disadvantages, which the construction of the femoral slideway proposed in DE 197 16 879 A1 of the applicant is designed to eliminate. The crux of this solution is to rotate the anterior or ventral cut in the transverse plane.
However, this more recent solution also requires improvement with respect to optimizing the joint function in cooperation with the collateral ligaments in particular, especially in order to reduce the load imposed thereon.
It is thus the object of the invention to disclose a femoral slideway with further improved function, as well as a knee endoprosthesis system that can be efficiently constructed and employed and has such a femoral slideway as its essential element, and finally an advantageous arrangement comprising femoral slideway and femur-size template.
This object is achieved in its first, foremost aspect by a femoral slideway with the features described in the present application.
The invention includes the essential idea that it is advantageous to prepare for a knee-joint replacement by resecting more bone from the femur dorsally than is replaced by the implant (the femoral slideway). The “diminution” of the femoral slideway thus brought about in the dorsal region, in comparison to the original dimensions of the (resected) condyles or to a femoral slideway fitted in the conventional manner, produces an effective reduction of the turning radius of the tibial plate belonging to the prosthesis system and hence reduces the load on the collateral ligaments.
The decrease in the dimensions of the femoral slideway measured between the outermost, dorsoventrally opposed points on the condyle shell surfaces, in comparison to the previously customary dimensioning, is preferably in the range between 2 and 5%. This is achieved by constructing the associated femoral slideway/femur-size template arrangement in such a way that the distance separating one or more pegs on the femoral slideway from its dorsal sliding surface is smaller by 5-15%, in particular by about 10%, than the corresponding distance by which bores in the femur-size template for positioning the pegs are separated from the contact surface that is to be apposed to the dorsal condyle surfaces of the femur.
The distance between the dorsal sliding surfaces and the one or more pegs on the inside of the femoral slideway is preferably in the range between 24 and 34 mm and in particular is 29 mm, the chosen value advantageously being kept constant in a knee endoprosthesis system for covering a relevant joint-size range.
As noted above, the femoral slideway/femur-size template arrangement is constructed in such a way that the distance between one or more pegs on the femoral slideway and its dorsal sliding surface is smaller than the corresponding distance between bores in the femur-size template and the contact surface of the template by 5-15%. and in particular about 10%. Stated differently, the corresponding distance between bores in the femur-size template and the contact surface of the template is larger by 5-15%, an in particular about 10%. than the distance between one or more pegs on the femoral slideway and its dorsal sliding surface. Accordingly, the distance between the bores in the femur-size template and the contact surface of the template can be between 26.4 mm and 37.4 mm (i.e., 10% larger than the range of 24 mm to 34 mm between the dorsal sliding surface and the one or more pegs of the femoral slideway). In another embodiment, the distance between the bores in the femur-size template and the contact surface of the template can be about 32 mm (i.e., about 10% larger than a distance of 29 mm between the dorsal sliding surface and the one or more pegs of the femoral slideway). In yet another embodiment, the distance between the bores in the femur-size template and the contact surface of the template can be between 30.45 mm and 33.35 mm (i.e., 5%-15% larger than a distance of 29 mm between the dorsal sliding surface and the one or more pegs of the femoral slideway).
Another distinguishing feature of the proposed femoral slideway is that particular dimensions maintain a largely constant relationship to one another, regardless of the size of the actual prosthesis. For instance, the ratio a:c between the maximal dorsoventral extent and the maximal lateral extent of the femoral slideway is about 0.9±0.02. The patellar pit formed between the condyle shells preferably has a depth b, measured from the dorsalmost point on the condyle shells, such that its ratio b:a to the maximal dorsoventral extent of the femoral slideway is in the range between 0.4 and 0.5, in particular is 0.44.
The patellar pit is thus lengthened in the dorsal direction, as a result of which the patella can be supported over a large area throughout its entire functional range of flexion.
This elongation of the patellar pit, which furthermore increases in accordance with the anatomy in implants of all sizes, allows for the fact that the patello-femoral contact surface in conventional femoral slideways has a relatively small bearing area. That is, in the region in which the patella leaves the trochlea and enters the intercondylar fossa, conventional femur components provide support only in the peripheral regions.
Furthermore, in the proposed femoral slideway the condyle shells are somewhat more strongly rounded in cross section (coronal section) than is the case in conventional femoral slideways. This modification was undertaken in the interest of improving the fit to the special tibia insert that belongs to a knee endoprosthesis system, but which is not within the scope of the invention.
The back surface of the femoral slideway, in one advantageous embodiment, bears a two-component Ti coating produced in a vacuum plasma procedure, consisting of a relatively thin, dense base layer and a severalfold thicker, open-pored cover layer. The dense base layer allows the femoral slideway, which for example consists of CoCrMo, to become completely sealed to the bone and, because it makes contact with the substrate over a large area, increases the stability of adhesion.
The open-pored and very rough surface of the cover layer provides ideal conditions for the growth of bony substance onto and into the carriage, producing a quasi “3-D interlocking” that can transmit pulling forces as well as pressure and transverse forces.
Additional advantages and useful features of the invention will be apparent from the subordinate claims and the following description of an exemplary embodiment with reference to the figures, wherein